Other suggestions I've had are to use Aloe Vera, and/or Slippery Elm to line the stomach. And Charcoal has been suggested. My prime worry at the moment is the diarrhea :(
Another similar rescue pony in my village was treated by same vet in the same way, and is now fine - however it is possible...
I'd love to turn him out with the others but he can't cope with the grass at the moment - it's a long haul and I take your point about the long term possible damage.
:( I 'rescued' a pony that was thin and had intermittent diarrhea. He's a cremello gypsy cob colt about 2 years old - Snowy. After putting him on a feeding/ worming program I realised he was not going to improve, and that he had not gained any weight. the vet started treatment for encysted...